Transistors, such as field effect transistors (FETs), having high breakdown voltages (e.g., above about 15 volts, and in some applications to about 80 volts or greater) are used in various integrated circuit applications. Transistors having high breakdown voltages may be referred to as high-voltage transistors, meaning that the transistors are configured to tolerate high voltages (e.g., above 15 volts).
One technique for creating a high-voltage transistor uses a lightly-doped diffusion region (LDD region) between the drain and gate of the transistor. This region is an active area of the transistor, and is sometimes referred to as a drain extension region. One class of devices utilizing LDD regions are Reduced Surface Field Devices (RESURF) devices.
High-voltage transistors may be configured with different drive currents. For instance, wide transistors may have higher drive currents than narrow transistors.
The active area of a high-voltage transistor is doped to form the LDD region, and there will be an optimum doping that will achieve a desired breakdown characteristic. Transistors having different drive currents may have different optimum doping concentrations relative to one another (e.g., wider transistors may have a different optimum doping concentration than narrower transistors), which can be problematic during fabrication of the transistors. Specifically, it can be problematic to utilize multiple doping steps during fabrication of high-voltage transistors in that multiple doping steps add process steps and thus may increase fabrication costs as compared to a single doping step. It would be desirable to develop alternative transistor structures, and methods of their formation, which enable a single doping step to appropriately dope multiple high-voltage transistors having different drive currents.